Multi-axis magnetic sensors or magnetometers, such as three-axis magnetic sensors, are particularly desirable for modern electronic compass applications. Known magnetoresistive (MR) sensors, such as AMR (anisotropic MR) sensors, GMR (giant MR) sensors, TGMR (tunneling GMR) sensors, and the like, however, can only detect magnetic flux that is parallel to the device plane and cannot detect flux that is perpendicular to the device plane. On the other hand, Hall-effect sensors can sense magnetic flux that is perpendicular to the device plane, i.e., along the Z axis, but cannot sense magnetic flux parallel to the device plane, i.e., in the XY plane. Thus complex geometric arrangements of these sensors are required in order to measure all three axes in a single device.
One of the most common types of magnetic field sensor is the well-known magnetoresistive (MR) sensor where, generally, the resistivity of the sensor varies according to a local magnetic field oriented in the same plane as the magnetoresistance. “Barber-pole” structures are added to allow a sensing of the magnetic field along one axis to include direction, or vector, information. Magnetoresistive sensors have been used successfully in electronic compass applications, using two sensors to detect the magnetic field in the same plane as the surface they are mounted on, (X, Y), with an additional sensor mounted in a particular way so that the sensitive element is properly aligned to sense the component of the magnetic field orthogonal (Z) to the plane of the system.
There are many known approaches to fabricating a magnetic sensor with three-axis sensitivities. One approach is to package a Z axis sensor of the same technology as the X and Y axis sensors in orthogonal disposition to the two-axis XY sensors. For example, three sensors are encapsulated separately before being soldered on a PCB as a module. In this case, the orthogonal (Z) axis sensor is mounted along the axis orthogonal to the PCB directly rather than along the plane, as in, for example, U.S. Pat. No. 7,271,586. This particular orthogonal axis sensor mounting, however, can be technically challenging, and significantly increases the cost of manufacturing, as well as results in an increase in the thickness of the final product.
Another approach uses two types of sensor technologies that are disposed on a common die with one constructed to sense vertical magnetic flux signals and the other constructed to sense horizontal magnetic flux signals.
Multi-axis sensitivities can also be achieved by building sensors on a sloped surface. For example, U.S. Pat. No. 7,126,330 describes a device where two magnetic field sensing devices are provided on a first surface to detect co-planar orthogonal X, Y axes and a third magnetic field sensing unit is disposed in a trench that is created in the first surface in order to detect the magnetic field in the Z axis. The '330 patent, however, is limited by the accuracy with which the inclined walls of the trench can be made so that they are at the same inclined angle.
There are disadvantages associated with each of the known approaches. For example, combining a Z axis magnetic field sensor, whose sensing direction is perpendicular to the device (XY) plane, with an X or Y axis magnetic field sensor(s) requires one or more additional packaging steps in order to install the Z axis magnetic field sensor vertically without significant angle variation. The additional packaging steps add significant cost to the whole product manufacturing process. Furthermore, variation in the positioning angle complicates signal processing since cross-talk signals from the XY plane are introduced if the Z axis magnetic field sensor in not perfectly vertical.
There is a need, therefore, for a low profile, inexpensive, but high performance, three-axis magnetic field sensor that can be produced in large volume using a simple manufacturing process.